This invention relates to devices for connecting communications equipment to a telephone line across an electrical isolation barrier.
In the past few years the internet has progressed from a computer curiosity to an integral part of business and commerce and education. The number of homes in the United States connected to the internet is estimated to exceed 40 million in the year 2001. Many new subscribers to internet services have come to realize that although a universe of information and products and services are available via the internet, the service connections available to most users are severely limited in speed, due either to limited modem speed or the limitations of the communications line serving the modem. The slow response time and downloading delays that are commonly experienced by internet users are continually frustrating to internet users, who are accustomed to the faster reaction times of systems such as video games, remote control selection of television programming, cell phone connectivity, and the like.
The internet has flourished on the promise of future services, such as video on demand, video conferencing for telecommuting and on-line meetings, high quality audio programming, multi-casting, and the like. These features cannot be provided on a real-time basis using existing internet connections.
One solution to this bottleneck is video cable service, which is available to over 90% of homes, with over 60% of homes in the United States subscribing to a CATV service. Although coaxial cable has very high bandwidth, a major drawback of cable TV systems is that they are predominantly non-switched and non-directed; i.e., every customer connected to the cable receives substantially all of the traffic on the cable. If cable systems are modified to carry individual channels of communication for each subscriber, it will be necessary to devise a time domain multiplex scheme for cable traffic, or a plurality of subcarriers, or some other multiplex network communications arrangement that can address each subscriber individually. Network communications could fill a large portion of the cable bandwidth, particularly if the cable systems continue to carry hundreds of TV channels as well.
Other communications technologies, such as fiberoptics, also offer the promise of xe2x80x9cunlimitedxe2x80x9d bandwidth. Although ten years ago there were predictions that every home would be connected to an optical fiber communications system by the year 2000, the magnitude of the task and the cost per home have combined to limit direct fiberoptic service to extremely high volume users. Rather, telephone companies have built fiberoptic networks across much of the country, but transfer the signals to the existing twisted pair copper wire circuits for transmission to the end user.
Ironically, the existing telephone system, unlike the cable TV systems and fiberoptic systems, does provide individually addressed communications circuits to each home (often more than one to each home). These circuits are typically formed by twisted pairs of copper wire, and are generally thought to have severe bandwidth limitations due to inductive losses over moderate distances, particularly as the frequency of the signal increases toward the upper end of the audio band. Indeed, plain old telephone service (POTS) provides voice circuits that are limited to 4 KHz bandwidth, and are terminated at the telephone company switch by an inductive terminator that enforces the 4 KHz limit. Standard analog modems must operate within this bandwidth restriction, and 56 Kbps appears to be the technical and physical limit to communications speed using this bandwidth.
A new technology has been devised to exploit the unused bandwidth that is available via the twisted copper pair network that now exists. Asynchronous Digital Subscriber Line (ADSL) takes advantage of the fact that a twisted copper pair may transmit a usable signal up at 1.5 Mbps for 2-3 miles from the telephone company switch, under normal conditions. ADSL permits simultaneous voice and internet traffic on the same twisted copper pair circuit by reserving the 0-4 KHz band for voice traffic. Bandpass filters, or xe2x80x9csplittersxe2x80x9d are placed in the circuit to separate the voice spectrum from the data spectrum. For data downloading, a plurality of subcarriers, each having a 4 KHz bandwidth, are transmitted in the 140 KHz-1 MHz band, each being independently modulated with the data signal and carrying up to 15 bits/sec per Hz. For data uploading, a plurality of subcarriers are transmitted in the 20-130 KHz band, the carrier bandwidth and modulation being similar to the upload spectrum. The technical details of ADSL are set forth in the ANSI T1.413 standard. A variant of this technology, termed Universal ADSL or DSL LITE, requires no splitter for voice service, and reduces the upload bandwidth to 140-550 KHz. This latter variant provides most of the advantages of DSL, such as high data rates for data uploads and downloads, without requiring the installation cost and hardware cost of a splitter at the customer premises.
At the receiver end of the telephone circuit, a DSL modem is connected to receive and transmit upload and download signals in the appropriate DSL format. The modem must provide a high bandwidth connection while also maintaining a required electrical isolation barrier between the telephone line and the user equipment. The baseline technology for transmission across an electrical isolation barrier is an isolation transformer.
In U.S. Pat. No. 5,500,895, issued to Gerald Yurgelites, there is described a method and apparatus for transmitting communications signals across an electrical isolation barrier. One example of this invention is the transmission of telecommunications signals across the isolation barrier that is required between a telephone network and any telephonic device (telephone, facsimile machine, modem, and the like). This invention employs paired capacitors connected across the isolation barrier, and differentially charges the paired capacitors to deliver the signal across the barrier without breaching the isolation that the barrier provides. In this arrangement, there is provided a line-side circuit that communicates with a user-side circuit through a plurality of paired capacitors. The separated circuits (sold as a two chip set) handle the transmit, receive, and tip/ring signals and convert them to differential charges for transmission across respective paired capacitors. The line-side circuit is powered by the telephone circuit, and the user-side circuit is powered by the user device (modem, fax machine, telephone, etc.).
Although this technology is superior to isolation transformer arrangements for virtually all previous applications, it is not well suited to DSL applications. It would be advantageous to eliminate the line-side device entirely, both to reduce cost and to reduce signal attenuation caused by the power requirements of the line-side device. However, there remains a need to convert the two-line telephone circuit to a four line transmit/receive circuit for proper signal handling.
The present invention generally comprises a high bandwidth telephone line transceiver for connecting telecommunications equipment to a telephone line across an electrical isolation barrier. A salient aspect of the invention is the elimination of isolation transformers, and the elimination of a line-side circuit for delivering signals to the user-side telecommunications equipment.
The invention provides one pair of isolation capacitors connected to the telephone line to transmit the telephone signals across the electrical isolation barrier, and provides at least 1000 VDC (up to 5000 VDC, depending on the jurisdiction) of isolation. The capacitors are connected to a user-side hybrid circuit that provides a line impedance match for the telephone circuit. The hybrid circuit also converts the two wire connection to the capacitors to a 4 wire circuit for separate transmit and receive signal paths.
The receive signal path derives a signal from one of the isolation capacitors, and feeds the signal through a signal op amp to a high pass filter that transmits the DSL download bandwidth and rejects the upload bandwidth and voice spectrum. The received signal is then fed to an analog/digital converter, and the digitized signal is connected to a digital signal processor, such as a DSP integrated circuit known in the prior art. The DSP circuit is connected to a modem, fax machine, network, or other data handling device via a USB port, a PCI interface, or the like.
The transmit signal path extends from the DSP circuit to a transmit path digital to analog converter. The resulting analog signal is fed through a low pass filter, which transmits the upload bandwidth and rejects the remainder of the spectrum. The signal then passes through a compensation network that maintains a flat impedance characteristic over the upload bandwidth, and then through a line impedance matching network to the other isolation capacitor that connects to the telephone line.
Due to the fact that the isolation capacitor impedance changes with frequency, particularly in the low frequency upload (transmit) band, it is necessary to employ the compensation network to flatten the impedance characteristic among all the subcarriers used in the upload band. The compensation network includes a balanced op amp that receives signal inputs from the low pass filter, and includes an RC feedback loop from each output to the respective input port. The RC values are chosen so that the output voltage of the circuit substantially tracks the line voltage of the telephone circuit across the isolation capacitors. This result is not affected by impedance changes with frequency, nor is it affected by differences in the ground voltage levels between the telephone circuit and the user equipment.
The hybrid circuit connects each receive signal input to a respective line out port through an RC series combination, and a resistor connects each receive signal input to the other line out port. This combination not only transmits the telephone signal to the receive signal inputs, it also cancels the near echo from the transmit portion of the circuit.
The high bandwidth transceiver of the invention eliminates the use of an isolation transformer, thereby eliminating a bulky component that radiates EMI like an antenna and generates heat. Transformers also may distort the telephone signal, due to inductance effects and possible magnetic coil saturation. The transceiver, on the other hand, may be realized as a small integrated circuit that may be surface mounted, with far less heat generation, far less EMI noise, and a far smaller area profile on a PCB.
The transceiver of the invention also eliminates the requirement for any electronic circuit to the telephone line side of the isolation barrier. All of the active components in the transceiver circuitry are connected only to the user side of the telephone line, with the exception of the pair of isolation capacitors that transfer the signals across the isolation barrier. There is no telephone side circuit drawing power from the telephone line. Thus the cost is diminished, and signal attenuation is also reduced.